Method of Determining a Grip Coefficient

A method of determining a grip coefficient at an interface between an inner tubular member and a gripping surface in a wellhead system, the wellhead system comprising a clamping arrangement comprising a collar having an externally tapered surface, the arrangement also including an annular component with an internally tapered surface, the collar and the annular component being relatively moveable between a first position in which the tapered surface of the annular component exerts no radial force on the collar and a second position in which the tapered surface of the annular component exerts sufficient radial force to distort the collar inwardly in order to grip the inner tubular member within the gripping surface and to support a load of the inner tubular member, the inner tubular member having an outer surface including a ridged profile, and the method comprising:

• (i) forming a test block having opposite side surfaces including a ridged profile representative of the outer surface of the inner tubular member, the test block being made from the same material as at least an outer surface of the inner tubular member;
• (ii) gripping the test block between two gripping plates by applying a gripping force normal to the side surfaces of the test block so as to form an interface between each of the side surfaces and a test face of a respective one of the gripping plates, the test face being made from the same material as at least an inner surface of the gripping surface;
• (iii) applying a gradually increasing load to the test block in a direction parallel to a plane of the interface;
• (iv) continuously monitoring relative movement between the test block and the gripping plate at the interfaces;
• (v) determining the applied load at which slip between the test block and the gripping plates occurs;
• and
• (vi) determining the grip coefficient by calculating the ratio of the applied load to the gripping force at the time slip occurs.